The present invention relates to the synthesis of mixed-metal catalysts, more particularly, high activity, long life, alcohol reforming catalysts, especially methanol, based upon nanosize Pt/Ru particles supported on an electroactive support, especially carbon.
Battery packs are currently the worldwide portable/emergency power source of choice for electrical devices. Researchers have long sought to develop small footprint fuel cells to replace rechargeable battery packs. Fuel cells offer efficient and direct conversion of the chemical energy stored in fuels to electricity in a very environmentally friendly (low polluting) fashion. In principle, fuel cells offer the potential to achieve higher power densities per unit volume, longer use times, and longer total equipment lifetimes than standard battery packs. Long term, this translates to lower cost, higher utility, and increased mobility.
For example, depending on device performance specifics, a battery pack for a laptop computer can provide ≈40-50 W-h of energy. If the laptop requires an average of 20 watts of power to run, then the battery pack can provide only 2-3 h of running time before requiring recharging. Although larger batteries can be used, one pays a price in weight and convenience (size). Furthermore, recharging requires access to a power grid. In contrast, a similar sized fuel cell based on methanol is anticipated to produce 50 W of power and last for 10-20 h before total methanol consumption. In this instance, replacing a used canister of methanol with another does not require access to a power grid (not rechargeable), provides instant continuity and saves weight if it replaces a second, backup battery. Finally, if lost or destroyed, a methanol canister will be easier to replace and much lower in cost than a high-tech, high-density battery pack.
The most efficient fuel cells use H2 as the reductant, and oxygen or air as the oxidant. The more advanced H2 based fuel cells can produce 0.8-1.0 A/cm2 at ≈0.7 V (0.5 W/cm2) with performance lifetimes measured in hundreds of hours. Unfortunately, the cost and weight required to store large quantities of gaseous H2, even as metal hydrides, are major drawbacks. Hence, fuel cells that use liquid hydrocarbon fuels, especially methanol (MeOH), are the focus of commercialization efforts.
Two of the more promising direct methanol fuel cell systems are the polyphosphoric acid fuel cell and the proton exchange membrane fuel cell (PEMFC). PEM based fuel cells are more convenient to work than polyphosphoric acids because they employ a solid acid electrolyte, e.g. Nafion(copyright) membrane.
The drawback to using MeOH as a fuel is that energy output can be much lower than hydrogen, typically in the 300-500 mA range at 0.5 to 0.3 V. For short runs, 0.8 A/cm2 at ≈0.5 V (0.25 W/cm2) have been achieved. In part, the lower performance is due to CO and/or methanol poisoning of the cathode due to crossover through the membrane (CO and MeOH compete with O2 for active catalyst sites). In part, this difference is due to the need to catalytically reform MeOH at the anode coincident with reacting the product hydrogen with oxygen, some efficiency is lost in the process. The methanol reforming reaction (1) is shown below:
CH3OH+H2Oxe2x86x92CO2+3H2xe2x80x83xe2x80x83(1)
For example, platinum metal by itself is an excellent catalyst for hydrogen fuel cells based on: 
However, CO (a typical impurity in many H2 sources) competes with H2 for active catalytic sites on Pt metal particles and readily poisons the catalyst. Thus, CO coverage of active catalyst sites limits the rate at which reaction (2) proceeds.
MeOH reforming, as shown in reaction (1), can actually proceed via two stepwise processes that can involve the formation of CO and/or CO2: 
The CO produced via reaction (4), is very effective in poisoning simple Pt catalysts. The actual problem lies in the fact that Pt metal alone is not an effective catalyst for the water-gas shift reaction, reaction (7), making CO difficult to remove from the surface. 
For the direct methanol fuel cell to be successful, an effective catalyst that promotes reaction (7) as part of the overall methanol reforming reaction is needed. Ruthenium is one of several metals that aid in promoting reaction (7).
Thus, improving the efficiency and activity of the MeOH reforming catalyst is desirable. A higher efficiency catalyst means less of the precious metal catalyst is required, and higher activity will minimize CO crossover poisoning of the cathode. It will be appreciated that there is a need in the art for highly active and efficient methanol reforming catalysts.
The present invention is directed to high activity, supported, nanosized mixed-metal catalysts, especially Ru/Pt catalysts for methanol reformation, and to methods of fabricating such catalysts. These methanol reformation catalysts are useful in methanol fuel cells, particularly portable, small footprint fuel cells such as polymer electrolyte membrane fuel cells (PEMFCs) that use methanol as a primary fuel source.
In a currently preferred embodiment within the scope of the present invention, the soluble metals are dissolved in a polyhydroxylic alcohol (polyol). The ratio of M1:M2:M3:M4 will typically vary from (0.001 to 1):(0.001 to 1):(0.001 to 1):(0.001 to 1). Presently preferred catalysts typically contain Ru and Pt, with or without additional metals. The ratio of Ru:Pt will typically vary from 0.001:1 to 1:0.001, and preferably from 0.1:1 to 1:0.1, and more preferably from 0.5:1 to 1:0.5. The polyols are preferably viscous alcohols to minimize diffusion and thereby prevent particle growth. Typical polyols used in accordance with the present invention include organic diols, triols, and tetraols. Ethylene glycol, glycerol, triethanolamine, and trihydroxymethylaminomethane are examples of currently preferred polyols. In the polyol process one has two choices, (1) make the colloid in the absence of support and then deposit it on the support or (2) make it in the presence of the support such that the support aids in minimizing particle growth. A typical example of each option is described below, realizing that variations of these examples can be made by persons having ordinary skill in the art.
Typical Colloid Preparation Procedure
An amount of metallic precursor (or precursors) is added to 100 mL of refluxing ethylene glycol. The reaction mixture is refluxed for 15 min. A first aliquot is taken out and quenched in water at ice-water bath temperatures. The quenched solution is centrifuged several times by decanting supernatant and washing with ethanol. A second aliquot is taken after 1 h and same workup process is applied. Samples are then vacuum dried overnight. These materials can then be redispersed in alcohol and deposited on a known amount of pretreated support material, such as carbon black.
Typical Supported Powder Preparation Procedure
An amount of metallic complex (M1 such as Ru complex) and an equivalent amount (by weight or mole) of a second metallic complex (M2, such as Pt complex) are dissolved in 10 mL of ethylene glycol, respectively. The two solutions are mixed and then added to a dispersion consisting of a weighed amount of support material, such as activated carbon, in 80 mL of ethylene glycol. The resulting mixture is refluxed and samples are taken after 15 minutes and 1 hour. Samples are quenched as above. The quenched solutions are centrifuged several times by decanting supernatant and washed with ethanol. Finally samples are vacuum dried overnight. The resulting catalyst is ready to use as is. Sometimes, it is flammable if kept from air during the preparation procedure.
The resulting catalysts include nanometallic powders on a support, bimetallic powders on a support, polymetallic nanopowders on a support, high surface area powders on a high surface area support, and low porosity metal nanopowders on a support.
The polyol solution is heated to a temperature in the range from 20xc2x0 C. to 300xc2x0 C. to reduce the metallic precursors to a zero valent state. If mixed with a support material, the mixed metallic catalyst particles form on the support material. The mixture is preferably heated to a temperature in the range from 60xc2x0 C. to 220xc2x0 C., and more preferably the mixture is heated to a temperature in the range from 70xc2x0 C. to 190xc2x0 C. The time required to heat the mixed-metal catalyst can vary, but the typical heating period generally ranges from 1 minute to 24 hours. Preferably, the heating period ranges from 1 minute to 5 hours, and more preferably, the heating period ranges from 1 minute to 1 hour.
The concentration of metallic precursors, such as Ru and Pt, affects the resulting mixed-metal catalyst particle size. Higher concentrations result in more particle growth and larger average particle size. The resulting mixed-metal catalyst particles typically have a particle size less than 1 xcexcm, preferably less than 0.1 xcexcm, and more preferably less than 0.05 xcexcm. Systems where the catalyst loading approaches that of the mass of the support are more prone to produce micron size metal particles on the support than nanometer size particles in support pores. Preferable loadings are less than 50 wt. % of catalyst to support weight. In addition, the amount of polyol used per gram of support/catalyst also affects the size of the metal particles simply on a dilution effect basis. Larger polyol volumes give smaller catalyst particles, even on the support.
For Pt/Ru catalysts, typical soluble metal species include, but are not limited to, PtCl2, H2PtCl6, Pt2(dba)3 (dba=dibenzylideneacetone), Pt(dvs) (dvs=divinyltetramethyl-disiloxane), Pt(Oac)2 (Oac=acetate), Pt(acac)2 (acac=acetylacetonate), Na2PtCl6, K2PtCl6, platinum carbonate, platinum nitrate, platinum perchlorate, platinum amine complexes, RuCl3.xH2O, Ru(acac)3, Ru3(CO)12, Ru(Oac)3, ruthenium nitrate, ruthenium perchlorate, ruthenium amine complexes, and mixtures thereof. Soluble platinum and ruthenium compounds are commercially available from a variety of vendors such as Strem Chemicals (Danbury, Mass.), Alfa Asear (Ward Hill, Mass.), and Aldrich (Milwaukee, Wis.). Other soluble metal species can be included in such as CuClx, Cu(Oac)x, CoClx, Co(Oac)x, and soluble Group 6, 7 and 8 metals. These metals can be used to aid in the water-gas shift reaction, reaction (7), or in forming stable metal hydrides for eliminating hydrogen, or to electronically modify the properties of the other metals in the mixed-metal catalysts.
Low metal loading on the support material is preferred. The soluble metallic precursors preferably have concentrations in the polyol sufficient to yield a metal loading on the support less than 100 wt. % metal to 100 wt. % support. More preferably, the metal loading on the support is less than 50 wt. % metal to 100 wt. % support, and most preferably less than 20 wt. % metal to 100 wt. % support. The catalyst loading is preferably in the range from 0.1 to 0.5 mg/cm2 of the support.
The support material preferably has a high surface area in the range from 10 to 2000 m2/g. More preferably, the support material has a surface area in the range from 200 to 1500 m2/g, and most preferably from 300 to 1500 m2/g. The high surface area is a result of an open porous structure. The support material preferably has pores sufficiently small to capture nanoparticles, but not too small to interfere with gas/liquid flow. Typical pore sizes will be in the range from 1 nm to 100 nm. Preferably, the pore size is in the range from 1 nm to 30 nm, and more preferably from 1 to 10 nm. Carbon is a currently preferred support material because of its high surface area and porosity, as well as its electrical conductivity properties. Other support materials can be used, including conductive metals and metal oxides such as indium tin oxide, silver, gold, Pt/Ag alloys, copper, and indium zinc oxides.
It will be appreciated that the present invention provides methods of preparing supported, mixed-metal MeOH reforming catalysts that are highly active and efficient.